The energy company may have issues closing its planned merger with Energy Transfer Equity because of a disagreement on the impact of tax issues on the deal.

Energy Transfer shares climbed more than 12 percent after an S-4/A filing with the Securities and Exchange Commission said that law firm Latham & Watkins told Energy Transfer that "it would not be able to deliver the 721 Opinion" that would make the deal tax-free for investors. Such filings contain data related to a merger or acquisition. The company's stock later gave up some of those gains and closed up 2.6 percent.

Williams also struggled after Democratic presidential candidate Sen. Bernie Sanders of Vermont said Monday that he is opposed to the company's proposed natural gas pipeline between Pennsylvania and New York and called on New York officials to reject the project.

"The possibility of methane leaks from the proposed Constitution Pipeline would be catastrophic to our air and our climate — and if this pipeline were approved, eminent domain would be used to seize land from farmers and homeowners," he said in a statement.

Williams has delayed the start up of the pipeline to the second half of 2017 from the fourth quarter of 2016.

Williams did not respond to a request for comment. Energy Transfer declined to comment beyond what was in the filing.